Deparment Of Pharmaceutical Analysis, C L Baid Metha College Of Pharmacy, Thoraipakkam.
Volatile impurities (VIs), primarily residual solvents and trace contaminants, are critical quality attributes that can significantly impact the safety, efficacy, and stability of pharmaceutical products. Owing to their high volatility and low boiling points, accurate detection and quantification of these impurities remain analytically challenging. Regulatory guidelines such as ICH Q3C, USP <467>, and BP prescribe stringent limits and classification of residual solvents into Class 1 (to be avoided), Class 2 (to be limited), and Class 3 (low toxic potential).This review presents a comprehensive overview of methodologies employed for the estimation of volatile impurities, with emphasis on sample collection, preparation, and analytical determination. Sampling techniques including static and dynamic headspace analysis, solid-phase microextraction (SPME), thermal desorption, and direct injection are critically discussed with respect to their applicability to diverse matrices. Advanced analytical platforms such as headspace gas chromatography (HS-GC), gas chromatography–mass spectrometry (GC-MS), and LC-MS/MS are highlighted for their sensitivity, selectivity, and suitability for trace-level analysis.Further, key aspects of method development and validation, including specificity, accuracy, precision, and robustness, are addressed in accordance with ICH Q2(R1) guidelines. The integration of optimized sampling strategies with advanced analytical techniques provides a reliable and regulatory-compliant framework for monitoring volatile impurities. This approach ensures product quality, patient safety, and adherence to global regulatory standards in pharmaceutical analysis.
Volatile impurities (VIs), often present as residual solvents or trace contaminants, pose significant risks to product safety, efficacy, and environmental integrity across pharmaceutical, chemical, and food industries. [1,2,3]. Their accurate detection and quantification are essential not only for ensuring consumer protection but also for meeting stringent regulatory standards such as ICH Q3C, USP <467>, and BP guidelines. These impurities, due to their high vapor pressure and low boiling points, are prone to loss or transformation during sampling and handling, making their analysis particularly challenging. [ 4,5]
To address these complexities, a range of specialized sampling and preparation techniques have been developed—each tailored to the volatility, polarity, and matrix characteristics of the target compounds. Techniques such as headspace gas chromatography (HS-GC), solid-phase microextraction (SPME), thermal desorption, and canister sampling offer robust solutions for isolating and analyzing volatile organic compounds (VOCs) from diverse matrices including solids, liquids, and gases. These methods are further supported by advanced analytical platforms like GC-MS and LC-MS/MS, which provide high sensitivity and specificity for trace-level impurity profiling.
This document outlines the materials, methods, and analytical strategies employed in the estimation of volatile impurities, emphasizing best practices in sample collection, preparation, and method validation. The goal is to establish a scientifically rigorous and regulatory-compliant framework for impurity monitoring in controlled environments. [6,7]
Volatile impurities are classified into three classes:
Figure :1 [9,10]
ICH QC3: VOLATILE IMPURITIES
|
CLASS 1 (AVOID) HIGHLY TOXIC/ CARCINOGENIC |
CLASS 2 (RESTRICTED) LIMITED USE ALLOWED |
CLASS 3 (LOW TOXIC POTENTIAL) |
|
Benzene |
Methanol |
Ethanol |
|
Carbon tetrachloride |
Acetonitrile |
Acetone |
|
1,2-dichloroethane |
Chloroform |
Ethyl acetate |
|
1,1-dichloroethane |
Toluene |
Heptane |
|
1,1,1-trichloroethane |
cyclohexane |
1-butanol |
Table:1 CLASS 1 SOLVENTS AND THEIR PROPERTIES:
|
SOLVENTS |
STRUCTURE |
OCCURRENCE |
LIMIT |
DETECTION METHODS |
|
Benzene |
|
contaminant in raw materials/solvents.
|
2 ppm |
(HS-GC), GC-MS |
|
Carbon tetrachloride |
|
by-product in chemical synthesis |
4 ppm |
HS-GC, GC-MS |
|
1,2-Dichloroethane |
|
in PVC production or chlorinated solvents |
5 ppm |
HS-GC, GC-FID, GC-MS |
|
1,1-Dichloroethene |
|
In residual monomer from PVC.
|
2 ppm |
HS-GC, GC-MS |
|
1,1,1-Trichloroethane |
|
In leftover from old solvent/cleaning agents |
10 ppm |
HS-GC, GC-FID |
Table :2 CLASS 2 SOLVENTS AND THEIR PROPERTIES:
|
SOLVENTS |
STRUCTURE |
OCCURRENCE |
LIMIT |
DETECTION METHOD |
|
Methanol |
|
Used as solvent in synthesis, extractions, and cleaning. |
300 |
HS-GC-FID, HS-GC-MS |
|
Acetonitrile |
|
Common in HPLC mobile phases, synthesis, recrystallization |
410 |
HS-GC-FID, HS-GC-MS |
|
Chloroform |
|
Used in organic synthesis, sometimes as a by-product. |
60 |
HS-GC-FID, HS-GC-MS, FTIR |
|
Toluene |
|
Reaction solvent, recrystallization solvent. |
890 |
HS-GC-FID, HS-GC-MS |
|
N,N-dimethylformamide |
|
Used in peptide coupling, polar solvent in synthesis |
880 |
HS-GC-FID, HS-GC-MS |
|
N,N-dimethylacetamide |
|
Solvent for polymers, synthesis. |
1090 |
HS-GC-FID, HS-GC-MS |
|
Cyclohexane |
|
Used in recrystallization, organic synthesis. |
3880 |
HS-GC-FID, HS-GC-MS |
|
Dichloromethane |
|
Extraction, reaction solvent, cleaning agent. |
600 |
HS-GC-FID, HS-GC-MS |
Table:3 CLASS 3 SOLVENTS AND THEIR PROPERTIES:
|
SOLVENT |
STRUCTURE |
OCCURRENCE |
LIMIT |
DETECTION METHOD |
|
Ethanol |
|
Used in formulation, extraction, and as a disinfectant |
5000 ppm (≤50 mg/day) |
HS-GC/FID, GC-MS |
|
Acetone |
|
Used as a cleaning agent, extraction solvent, and in chemical synthesis |
5000 ppm (≤50 mg/day) |
HS-GC/FID, GC-MS |
|
Ethyl acetate |
|
Used in extraction, chromatography, and as a flavoring agent |
5000 ppm (≤50 mg/day) |
HS-GC/FID, GC-MS |
|
1-propanol |
|
Used as a solvent in the pharmaceutical industry for drug formulation. |
5000 ppm (≤50 mg/day) |
HS-GC/FID, GC-MS |
|
2-propanol |
|
Used as a solvent and disinfectant in pharmaceuticals |
5000 ppm (≤50 mg/day) |
HS-GC/FID, GC-MS |
|
Heptane |
|
Extraction solvent and cleaning agent |
5000 ppm (≤50 mg/day) |
HS-GC/FID, GC-MS |
MATERIALS AND METHODS:
SAMPLE COLLECTION:
Volatile impurities are collected using methods that prevent the loss of these compounds during sampling and transport, before analysis by gas chromatography (GC) or mass spectrometry (MS). The best method depends on the sample matrix, the specific volatile organic compounds (VOCs) of interest, and the required sensitivity.
The samples used in the estimation of volatile impurities for each classes are:
Class 1 samples: APIs, Drug formulations, Excipients.
Class 2 samples: APIs, Finished drug products, Biopharmaceutical intermediates, Coatings and flavourings.
Class 3 samples: Pharmaceutical formulations, Topical formulations, Injectables & inhalations, Excipients.[11]
GENERAL CONSIDERATION OF SAMPLING OF VOLATILE IMPURITIES:
Regardless of the method chosen, several factors are critical for obtaining accurate and representative samples of volatile impurities are:
For class 1 solvents:
*Store cool (≤ 25°C) and under inert atmosphere (e.g., nitrogen)
*Use tightly sealed amber glass or metal containers
*Place in explosion-proof flammable chemical cabinets.
For class 2 solvents:
*Store at controlled room temperature (15–25°C)
*Ensure well-ventilated storage area
*Prefer amber containers to protect from light.
For class 3 solvents:
*Store at room temperature (20–25°C)
*Use airtight standard containers
*Avoid exposure to heat and direct sunlight.[12]
THE METHODS USED FOR THE SAMPLE COLLECTION OF VOLATILE IMPURITIES ARE:
Table:4
|
METHOD |
SUITABLE SAMPLE TYPE |
|
Static Headspace (HS) |
APIs, Tablets, Capsules, Syrups, Injectables |
|
Dynamic Headspace (Purge-and-Trap) |
Aqueous solutions, Low-level volatiles, Environmental samples |
|
Solid-Phase Microextraction (SPME) |
APIs, Tablets, Creams, Gels, Biological fluids |
|
Direct Solvent Extraction |
Excipients, Syrups, Semi-solids, Oily matrices |
|
Thermal Desorption |
Polymers, Coatings, Topicals, Solid dosage forms |
|
Direct Injection / Dilution |
Liquid formulations, Injectables, Solvent-based products |
|
Gas Sampling (Sorbent tubes / Canisters / Bags) |
Inhalation products, Aerosols, Air/Vapor samples. |
Each sample collection method is selected based on the sample type, volatility of the impurities, and regulatory requirements. In that the Volatile impurities are extracted from samples using the techniques that suit the type of sample and the class of solvent being analyzed.
For drug substances, drug products, and excipients,especially those containing Class 1 or Class 2 solvents like benzene or methanol, the headspace sampling and solid-phase microextraction (SPME) methods are commonly used. These methods allow volatile compounds to be released into the gas phase and captured without altering the sample. For intermediates and solvent mixtures, direct aqueous injection may be used when the solvents are water-soluble. For Packaging extracts and environmental samples, which may contain Class 1 or Class 2 solvents like carbon tetrachloride or THF, are often analyzed using thermal desorption or canister sampling method.For Class 3 solvents, these are considered to have low toxic potential and are generally regarded as safe when present in pharmaceutical products at levels below 5000 ppm. Solvents like ethanol, acetone, isopropanol, and butanol. These solvents are often used in formulation processes, and their presence is typically monitored in final dosage forms such as tablets, capsules, and oral liquids.Extraction of Class 3 solvents is usually performed using headspace sampling or direct aqueous injection, depending on the matrix and volatility.
SAMPLE PREPARATION:
Sample preparation for volatile impurities involves selecting a technique to isolate and introduce the volatile compounds to an analytical instrument, such as a gas chromatography.[20]
It involves selecting suitable volatile solvents for dissolving the sample, cleaning the sample to remove particles, and using techniques to increase or isolate the volatility of the desired components, such as evaporation through nitrogen blowdown or using the headspace method. Other methods include purge-and-trap, extraction, and derivatization to convert non-volatile compounds into volatile ones for analysis, often by gas chromatography.[21,22]
The choice of sample preparation depends on the sample matrix (solid, liquid, or gas), the concentration of impurities, and the specific impurities of interest. GC analysis requires the analyte to be in the gas phase before entering into the column.
The goal is to obtain a pure, volatile sample in a form that can be directly injected into the instrument for analysis, often after dilution in a volatile solvent. [23]
COMMON SAMPLE PREPARATION TECHNIQUES:
1.SOLVENT EXTRACTION:
It Involves dissolving the sample in a suitable volatile solvent (like hexane or methanol) to extract the volatile impurities. This is a common method for both solid and liquid samples.[24,25,26]
2.SOLID PHASE MICROEXTRACTION:
A solvent-free technique where a coated fiber is exposed to the sample to absorb and concentrate target analytes, which are then desorbed directly into the GC.[27]
3.HEADSPACE ANALYSIS:
There are two types of headspace analysis techniques are;
1.Static Headspace: The sample is placed in a sealed vial and heated to vaporize the volatile compounds, which collect in the empty space (headspace) above the sample.
2.Dynamic Headspace: Volatile compounds from the sample are concentrated onto a sorbent trap by pumping the headspace gas through it.[28,29,30]
4.PURGE-AND-TRAP TECHNIQUE:
This method is Similar to dynamic headspace but often used for aqueous samples, where a gas is bubbled through the sample to remove volatile organic compounds (VOCs), which are then captured on a sorbent trap.[31,32,33]
Figure :2 [34,35,36,37,38]
ANALYTICAL APPROACHES FOR VOLATILE IMPURITIES:
Analytical approaches for volatile impurities refer to the systematic methods used to detect, identify, quantify, and control trace-level volatile compounds. It begins with selecting the most suitable technique based on the volatility, polarity, and nature of the sample matrix.[39]
Appropriate sample preparation strategies such as headspace analysis, solid-phase microextraction (SPME), or direct injection are then applied. Method development focuses on optimizing key parameters like temperature, carrier gas flow rate, and column selection to achieve the best resolution. [40,41]
Each method undergoes validation to confirm specificity, sensitivity, accuracy, and robustness, ensuring reliable results. Finally, the approach must align with regulatory guidelines such as ICH, USP, and BP standards to meet industry compliance requirements. [42,43,44]
Headspace GC (HS-GC) is used for quantifying residual solvents with matrix-free sampling, while GC-MS provides high specificity for identifying unknown impurities. SPME-GC enables solvent-free, non-invasive sampling of trace volatiles, making it suitable for packaging and environmental studies. Thermal Desorption GC allows direct analysis of VOCs from materials or containers without sample preparation, and LC-MS/MS is applied for semi-volatile or polar impurities, especially genotoxic or reactive volatiles. Together, these methods offer a comprehensive and reliable framework for monitoring volatile impurities across industries. [45,46,47,48,49,50]
Table :5
CONCLUSION
The conclusion of the analytical approach for the estimation of volatile impurities:
Figure :3 [51,52,53,54]
The estimation of volatile impurities (VIs) are critical for ensuring product safety and regulatory compliance, particularly under ICH Q3C and USP <467> guidelines. [55,56,57]
Among the most widely adopted techniques, headspace gas chromatography (HS-GC) stands out for its robustness and minimal sample preparation, with flame ionization detection (FID) or mass spectrometry (MS) offering reliable quantitation and compound identification. Solid-phase microextraction (SPME) coupled with GC-MS provides a solvent-free, highly sensitive alternative, especially effective for trace-level detection in complex matrices. Thermal desorption GC-MS further enhances sensitivity for ultra-trace impurities, particularly in air-sensitive applications. [58,59,60,61]
Direct aqueous injection (DAI), though less commonly used, remains relevant for water-soluble volatiles. Method development must account for matrix effects, analyte polarity, and detection limits, with validation parameters aligned to ICH Q2(R1) standards. These techniques, when strategically selected and optimized, from the backbone of impurity profiling in regulated environments. [62,63,64]
REFERENCE
T. Anitha, Dr. Vijayageetha, Analytical Approaches for the Estimation of Volatile Impurities, Int. J. of Pharm. Sci., 2026, Vol 4, Issue 4, 2269-2281, https://doi.org/10.5281/zenodo.19590957
10.5281/zenodo.19590957
